1. Field of the Invention
The present invention relates to a head access mechanism for moving a head, which is suitably used in recording/reproducing apparatus such as floppy disk and hard disk units for high-density recording.
2. Description of the Related Art
Conventional recording/reproducing apparatus such as floppy disk and hard disk units for high-density recording comprise a linear actuator in a head access mechanism for moving a magnetic head in the radial direction of a recording disk. As such a linear actuator, a low-profile actuator shown in FIGS. 1A and 1B is proposed in Japanese Unexamined Patent Disclosure No. 61-196751. The linear actuator shown in FIG. 1A comprises fixed magnetic field generating section 10 and a carriage section 20 as a moving section.
Generating section 10 is fixed and comprises an E-shaped yoke 14 including central extending portion 19 and side extending portions 18a and 18b, an I-shaped yoke 12 connected to the ends of portions 19, 18a, and 18b. Yokes 12 and 14 comprise a closed magnetic circuit. Permanent magnets 16a and 16b are arranged parallel to side extending portions 18a and 18b so as to oppose central extending portion 19.
Carriage section 20 comprises case 34, coil 22 having both its upper and lower ends fixed to case 34, and magnetic heads 24 and 26 respectively arranged on sides 0 and 1. Central extending portion 19 extends through the central hole of coil 22 without contacting each other. A magnetic field generated by coil 22 in accordance with a supplied current interacts with a magnetic field generated in central extending portion 19 by magnets 16a and 16b, and generates a propulsive force. Generating section 10 and coil 22 comprise a voice coil motor. Case 34 has a box-like shape, and can house most of generating section 10. Magnetic head 24 is arranged on arm portion 32 extending along the same plane as that of upper plate 36 of case 34 on a side opposite to generating section 10. Support 28 is mounted on upper plate 36. Arm portion 30 having magnetic head 26 is mounted on the distal end of support 28 so as to be freely rotated in the vertical direction. Magnetic head 26 on side 1 is attached to arm portion 30 so as to oppose magnetic head 24 on side 0.
As described above, generating section 10 and coil 22 comprise the voice coil motor. Carriage section 20 is linearly driven in accordance with a current supplied to coil 22. Rotary bearings 38a, 38b, and 38c are attached to the outside of case 34. Rotary bearings 38a to 38c are guided by fixed guide rails 40a and 40b so that case 34 and hence magnetic heads 24 and 26 are linearly moved in the radial direction of recording disk 42.
In the linear actuator having such a structure, since most of generating section 10 can be housed in case 34, the overall height can be reduced, and hence a low-profile actuator can be realized. In addition, since case 34 has a box-like structure, a sufficient mechanical strength can be obtained, and moreover, an excellent resonance frequency characteristic (high resonance frequency) can be obtained.
In such a conventional linear actuator, however, heads 24 and 26, arm portion 30, and support 28 are located above upper plate 36 of case 34, as shown in FIG. 1B. For this reason, center of gravity G of carriage section 20 is located above central position T of coil 22 in the vertical direction. When a magnetic field generated by coil 22 in accordance with a supplied current interacts with a magnetic field generated in central extending portion 19 of generating section 10, a propulsive force obtained by the interaction is applied to central position T of coil 22. When carriage section 20 is moved by propulsive force Ft, inertia force vector Fg acting on center of gravity G is not present on the same line as that of the vector of propulsive force, i.e., propulsive force vector Ft. Therefore, moment M is generated around center of gravity G. This causes pitching when carriage section 20 is linearly moved, so that a resonance point is generated in a low-frequency band because of this pitching. As a result, when a servo control is employed to position the head, the system for the servo control becomes unstable, or oscillates due to the low-frequency resonance point. Also, for example, the contact state of each of the magnetic heads 24 and 26 with the recording disk 42 becomes unstable. Therefore, this poses a problem when a magnetic head must be positioned on a destination track of a magnetic disk with high precision.